High voltage batteries with voltages of up to 400 volts, or higher, when fully charged, are employed for on-board energy storage in hybrid or electric vehicles.
These batteries are assemblies of multiple cells, which, in current practice often employ a Li-ion (lithium ion) chemistry. These cells, which individually develop about 4 volts, are at least connected in series to develop the required voltage and may optionally be connected in parallel to develop the required energy storage capacity. Each cell-to-cell connection should have the lowest possible electrical resistance to minimize the overall internal resistance of the battery and enhance its performance Most commonly ultrasonic welding, a process for making a solid state weld without melting the workpieces is a preferred joining process.
Batteries are progressively assembled and constructed by sequential addition of a number of smaller entities and require electrical interconnections. The starting point is the fabrication of anodes and cathodes by the laying down of appropriate materials on thin foil metal current collectors which range in thickness from about 10 to 20 micrometers thick. The anode material is often graphite-based and laid down on a copper current collector. The cathode may be lithium manganese oxide (LiMn2O4), or other manganese-containing compounds like nickel-manganese-cobalt oxide Li(NixMnyCoz)O2, or lithium aluminum manganese oxide (LixAlyMn1-yO2) and laid down on an aluminum cathode. Normally the current collector is more extensive than the electrode, leaving a portion of the metal foil current collector extending beyond the electrode area.
A pouch cell is formed by stacking a spaced apart series of anodes and cathodes with interposed electrolyte-soaked separators in facing relation and sealing them within a soft-sided container. All of the anode current collectors may then be gathered together and attached, as a group, to an anode tab which may be about 200 micrometers thick. Similarly, all of the cathode current collectors may be gathered together and connected to a cathode tab, which, again, is about 200 micrometers thick. Commonly the cathode tab is aluminum for compatibility with the cathode foils but the anode tab may be copper or nickel or nickel-plated copper.
Therefore, the pouch cell may contain a plurality of Li-ion cells, connected in parallel. Commonly three pouch cells are connected in parallel by welding together their anode or cathode tabs, often to a busbar or like current-carrying connector. By connecting these groups of three pouch cells in series modules may be produced, with the modules, in turn, being assembled into a battery pack.
The tabs for each cell are thin sheets, about 200 micrometers thick which may be up to about 45 millimeters in width. In fabricating a pouch a large number, typically up to about 20, and sometimes in excess of 60, current collector foils, are welded to one another and to the tab. In current practice, during the pouch welding operation, a workpiece stack, comprising the current collector foils and the tab, is clamped between the opposing faces of an ultrasonic welder. The tool faces are then ultrasonically excited, inducing the current collector foils to slide back and forth relative to one another at high frequency, usually from about 20 to 40 kHz. This high frequency sliding generates heat and fragments and disperses the oxides and surface films between the workpieces to expose fresh metal surface and enable a metallurgical bond without melting the materials.
A similar procedure may be followed when parallel-connecting the pouches. In this case, the tabs will be about 200 micrometers thick and the busbar may be about 500 to 1000 micrometers thick, resulting in a somewhat thicker workpiece stack.
The ultrasonic welding tool faces may be square or rectangular in plan view and may be textured or knurled. Typical tool dimensions, and hence weld dimensions may be about 4-10 millimeters on a side, with a 10 millimeter wide by 4 millimeter high tool being common. Several such welds are commonly used, spaced generally equal distances apart and arranged end to end across the foil width.
In service, these foils and welds are subject to appreciable loadings, both mechanical and thermal. Under these loadings one or more of the weld patches, or the zone immediately adjacent to the weld patch may develop tears and/or cracks, reducing battery capability and over time, promoting battery failure. There is therefore need for additional welding and joining processes for battery cell interconnections, both foil to tab and tab to busbar, to produce lower resistance interconnections and stronger joints. Such welding and joining processes are particularly important for lithium-ion cells extensively employed in high voltage vehicle propulsion batteries.